How Does Water Quality Impact Fisheries?
“We forget that the water cycle and the life cycle are one.”
-Jacques Yves Cousteau
Quoted in the Smithsonian’s Water/Ways exhibit coming to Sanibel this fall, the late undersea explorer Jacques Cousteau’s reminder of the inextricable link between water and life raises questions regarding the future of sustainable fisheries.
Southwest Florida locals, from fishermen to conservationists, have speculated that freshwater discharges from Lake Okeechobee, pollution, and red tide are causing a decline in our fisheries. This is particularly true of bay waters along Sanibel Island and south to Estero Island, where flow comes from the Caloosahatchee River. But the science to prove the speculations is scarce.
But, the ear stones of spotted seatrout may unlock one piece of evidence as to how water quality in local waterways affects fisheries. The trout, unlike most fish and shellfish, lives its entire life in the estuary.
“Their otoliths – a balance mechanism, ear stones -- have growth rings like a tree,” says Dr. Eric Milbrandt, director of the Sanibel-Captiva Conservation Foundation (SCCF) Marine Laboratory at the J.N. “Ding” Darling National Wildlife Refuge’s Tarpon Bay on Sanibel Island.
SCCF’s spotted seatrout otolith studies in 2006 and 2011 correlated that low salinity in the estuaries due to influxes of fresh water caused a slowing of the seatrouts’ growth patterns. “They are not able to eat as much because they are expending more energy compensating for the low salinity,” Milbrandt explained.
Local inshore waters support only two commercial fisheries – mullet and blue crab. Since recreational fishermen are not required to report catches, it becomes difficult to determine population fluctuations of noncommercial catches.
“A lot of fisheries biologists [in Florida] don’t take a bottom-up approach,” said Milbrandt. “They’re more interested in mortality and how much of that is from harvest … instead of focusing on drivers.” Conditions such as water flow, salinity, turbidity, color, seagrass leafing, and shade are indicators and impactors.
“The approach we focus on is based on nursery habitat…,” said Milbrandt. “We found that the [water] quality has been in decline since we started studying in detail.”
Salinity is the big factor, says Joy Hazell, the local Florida Sea Grant agent. Salinity levels are low this year because of Lake Okeechobee releases, but also as a result of a rainy winter. “And we’re not going to see results immediately from those releases because the adult fish can get out of the way,” she said. “The juveniles that can’t get away, in a few years might end up affecting populations.”
The Florida Fish and Wildlife Conservation Commission (FWC) studies a different fish to gauge the health of fisheries along the Caloosahatchee River, says fish biology research scientist Dr. Phillip Stevens, now stationed at the St. Petersburg research institute branch, but formerly at the Charlotte Harbor Field Laboratory. His studies focused on the endangered smalltooth sawfish, which is easy to tag because of its size.
Smalltooth sawfish. Photo courtesy of "Ding" Darling Wildlife Society-Friends of the Refuge.
“We studied the effects of freshwater inflow for about five years into the Caloosahatchee - how fish changed distribution not just when there was too much water [discharge] but also when there was not enough and there was drought,” he said. The sawfish moved up and down river to adjust to the water levels, he said.
Hazell reports that biologists are not seeing a current decline in fish populations, and cites an unusually high stone crab harvest this past season and a recent high bay scallop local count by the FWC.
Stevens confirms that report with sawfish. “We’ve been studying the sawfish for 12 years and the populations have been pretty steady and even increasing slowly…. We never saw any mortality except from cold events.”
“The biggest challenge is pure volume of fresh water not as much the quality, unless we get algal bloom,” Hazell said. “Brown water doesn’t mean dirty water; that can be caused by tannins. But nitrogen and phosphorus in the water, there’s a direct link to affecting fish populations because of oxygen depletion. The fisheries are resilient as long as it’s not an ongoing, continuing thing.”
Stevens points out, consistent with SCCF’s findings, that stress may affect fish growth rate; his research currently concentrates on that. He also watches for other adverse effects such as lesions, parasites, and infections.
“Our goal is to make sure populations stay at a high enough level that the fish can be resilient to disturbances [hurricanes, red tide, high flow, and cold],” he said.
“The health of our waterways and fisheries is crucial to the health of our wildlife in general as well as human populations,” said Toni Westland, supervisory refuge ranger at “Ding” Darling Refuge. “To raise awareness of water issues locally and worldwide, the refuge is hosting The Smithsonian’s Water/Ways exhibit this fall.”
The free exhibit takes place in the refuge’s Visitor & Education Center Oct. 28 through Dec. 8, 2016. It features lecture and film series, art and photography exhibitions, and a Kids Fishing Derby.
Click here to learn more.